The invention relates to an input video processor circuit and particularly, to an input video processor with a gain control circuit, which independently manipulates the master gain and the individual gain of different channels while providing blanking, black level clamping and white shading.
Prior art color television cameras require various circuits for manipulating the gains of the red, green, and blue channels by similar amounts while allowing manipulation of the master gain of the processor over a selected range. In addition, further circuitry is required to provide black level clamping, blanking prior to clamping to provide a clean pulse level, and white shading adjustments to compensate for the non-uniformity of the camera tube sensitivity. In instances where the light available to a camera begins to decrease and the iris of the camera cannot compensate for the reduction of light, gain control circuits are utilized to increase the electronic gain of the system. Typical prior art video processor circuits of cameras presently available, allow the adjustment of electronic gain in two or three steps using for example zero, 6 decibels (db) which is a factor of two, and 12 db which is a factor of four.
A classic problem in typical input video processors, is that referred to as clamp noise. This is caused by the fact that cameras normally use high speed horizontal rate clamps to try and stabilize the black level to remove hum and tilt from the picture due to AC couplings, and the like. In so doing, the inherent noise that is generated by the preamplifier is converted to undesirable low frequency noise by the sampling system.
By way of example only, a typical prior art input video processor circuit is utilized in the Ampex color camera model number BCC-10, described in the Ampex Manual Broadcast color camera system, catalog #1809339-01, August 1979.